Titanium dioxide is classified into two representative crystal types; rutile type and anatase type. As a sol of fine particles of a metal oxide for a high refractive index, a material having fine particles of an anatase-type titanium dioxide having a refractive index of no=2.56 and ne=2.49 as a main component has been mainly used up to now. On the other hand, a rutile-type titanium dioxide has a refractive index of no=2.61 and ne=2.9 (no: refractive index with respect to ordinary ray, ne: refractive index with respect to extraordinary ray) (Courses in Experimental Science, compiled by The Chemical Society of Japan), and is known for its excellence in optical features such as a high refractive index, ultraviolet absorption and the like, as compared to the anatase type. So, synthesis of fine particles of a rutile-type titanium dioxide and the sol thereof has been actively attempted. However, in reality, fine particles of a rutile-type titanium dioxide and a sol which can be industrially used have not yet been obtained.
For example, in a method for preparing an anatase-type titanium dioxide of a low refractive index type by nature, there has been reported a method comprising adding tin as a doping agent to forcibly change it into a rutile type. According to the method as described in Chem. Mater., Vol. 7, p. 663 (1995) by H. Cheng et al., synthesis is carried out under the conditions including strong acid and high concentration by a hydrothermal method. However, there are drawbacks such that a crystal grain diameter exceeds 20 nm due to high temperature of 220 degree centigrade and a good rutile-type titanium dioxide is not obtained because tin dioxide is mixed. Furthermore, according to the method as described in Nano Mater., Vol. 4, p. 663 (1994) by X. Z. Ding et al., an anatase type is converted into a rutile type by having a mixed solution of titanium tetrabutoxide dispersed in water and ethanol as a raw material, and adding tin (IV) chloride pentahydrate as a doping agent and hydrochloric acid as a catalyst under a temperature condition of 60 degree centigrade. However, in this method, there is a drawback in that the anatase type remains or a tin oxide is generated. In Patent Document 1, an anatase type is usually changed to a rutile type by adding a tin compound, but there is also the same drawback.
On the other hand, a method for synthesizing a rutile-type titanium dioxide at a low temperature has been reported in Jpn. J. Appl. Phys., Vol. 37, p. 4603 (1998) by H. D. Nam et al. However, according to this method, there is generated an agglomerate having an agglomerated particle diameter of 200 to 400 nm in which long-fibrous rutile-type titanium dioxides are gathered.
On the other hand, a diethylene glycol bisallylcarbonate resin (refractive index: 1.50) of a low refractive index has been used for plastic ophthalmic lenses. However, there have recently been developed a resin lens (refractive index: 1.60 to 1.70) obtained by thermopolymerizing a thiol compound and an isocyanate compound for forming a thiourethane bond as described in Patent Document 2, and a resin lens (refractive index: not less than 1.70) obtained by forming an epithiosulfide bond according to ring opening thermopolymerization of a thioepoxy compound as described in Patent Document 3. However, since plastic lenses have a drawback of easily causing damage due to low scratch resistance, a method comprising preparing a coating liquid using silica sol and an organic silicon compound, and providing a hard coat film on a surface has been carried out.
Meanwhile, since a light curable hard coat film can be easily formed, it has been used as a hard coat film for improving scratch resistance in the same manner.
Furthermore, plastic lenses have a drawback in that impact resistance is low. For that reason, there has been used a method comprising providing a primer film between a substrate and a hard coat film for absorbing impact. However, since the coat film has a low refractive index while the substrate has a high refractive index, an interference fringe appears on the coat film due to the difference in the refractive indexes, causing a problem of bad appearance. For that reason, the refractive index of the hard coat film, the primer film or the light curable hard coat film also needs to be close to the level equivalent to the refractive index of the substrate.
Likewise, a fine particle having a high refractive index which is excellent in photo-durability, weather resistance and the like, a sol thereof and a coating liquid has also been demanded in the fields of plastic deterioration preventive additives, a cosmetic additives, optical members such as a camera lens, an automobile window glass, a plasma display, a liquid crystal display, an EL display, an optical filter and the like, products of a metal material, a ceramics material, a glass material and a plastic material used for adjusting a refractive index, and the like.
Antimony oxide has been recommended as fine particles of a metal oxide and a sol to be added to a coating liquid for obtaining a coat film having a high refractive index up to now. However, since the refractive index of a plastic substrate of a lens is recently not less than 1.6, it is not already possible to use antimony oxide. The antimony oxide itself has a refractive index of 1.7, but it is filled up in an organic silicon compound having a low refractive index or the like, prior to use. Therefore, the refractive index as a coat film becomes even lower than that of a substrate.
As a means to solve this problem, at present, there has been used a method such that fine particles including an anatase-type titanium oxide having a higher refractive index than that of antimony oxide or the like are contained in a hard coat film, a primer film and a light curable hard coat film.
However, it was found that a coat film using fine particles of an anatase-type titanium dioxide is inferior in photo-durability. That is, by a photo-catalytic action of titanium dioxide, the decomposition of an organic substance occurs due to electron-holes generated by photo-absorption, resulting in problems of scratch resistance, surface hardness, abrasion resistance, transparency, heat resistance, photo-durability, weather resistance, ultraviolet interrupting property and the like.
At present, for the purposes of improvement in an interference fringe and photo-durability of a coat film containing fine particles of an anatase-type titanium dioxide, there have been applied, for example, composite fine particles of an anatase-type titanium dioxide and a metal oxide, fine particles of an anatase-type titanium dioxide coated with a metal oxide, a coating liquid using such fine particles or a coat film, as described in Patent Document 4.
All of them aim to achieve inactivation of fine particles of an anatase-type titanium dioxide coated with a metal oxide. By coating fine particles of titanium dioxide with a metal oxide as described above, photo-durability is improved. However, since titanium dioxide in use is an anatase type, a refractive index is about 2.5. When fine particles are coated with a metal oxide in order to enhance the photo-durability, the refractive index is greatly lowered such that it is lower than a refractive index inherent in an anatase-type titanium dioxide. So, the effect for enhancing a refractive index of a coat film is low. Even though the refractive index is enhanced by reducing the amount of a metal oxide to be used for coating, photo-durability becomes insufficient. So, when fine particles are used for a high refractive index substrate, and particularly, an ophthalmic lens substrate having a refractive index of not less than 1.70, in reality, there are some difficulties in enhancing a refractive index of a coat film while maintaining photo-durability so that an interference fringe cannot completely be removed.
On the other hand, in reality, fine particles of a rutile-type titanium dioxide having a refractive index higher than that of the conventional anatase-type titanium dioxide and the sol thereof have not been used as described above.
Patent Document 1: Japanese Patent No. 2783417
Patent Document 2: Japanese Patent Laid-open No. 1997-110956
Patent Document 3: Japanese Patent Laid-open No. 2002-194083
Patent Document 4: Japanese Patent Laid-open No. 2001-123115